A wind turbine for wind power generation includes, for instance, a tower installed on the ground, a nacelle placed atop the tower, a rotor head rotatably attached to the nacelle, and a plurality of blades attached radially to the rotor head. The rotor head is connected to one end of a main shaft. The other end of the main shaft extends within the nacelle. The main shaft is rotatably supported by a main bearing provided in the nacelle. A step-up gear provided in the nacelle is connected to the other end of the main shaft and the step-up gear is connected to a generator. Upon rotation of the rotor head by wind against the blades, the rotational force is inputted to the generator via the main shaft and the step-up gear so as to generate power by the generator.
In the main bearing and the step-up gear, wear of slide surfaces could lead to generation of abnormalities. In view of this, Patent Literature 1 proposes an abnormality detecting apparatus for detecting abnormalities of bearings. This abnormality detecting apparatus includes a vibration sensor, a frequency analyzing part, a memory part and an abnormality detecting part. The frequency analyzing part analyzes a frequency of a vibration signal detected by the vibration sensor and detects a natural frequency. The memory part stores the detected natural frequency. More specifically, the vibration signal is converted to a Fourier spectrum by Fourier transformation and then, based on the Fourier spectrum, the natural frequency is detected. The abnormality detecting part detects abnormalities of the bearing based on change in the detected natural frequency.